Electrodiffusion in ionic channels of biological membranes

An important class of biological molecules - proteins called ionic channels conduct ions (like Na+, K+, Ca++ and Cl-) through a narrow tunnel of fixed charge. Ionic channels are the main pathway by which substances move into cells and son are of great biological and medical importance: asubstantial fraction of all drugs used by physicians act on channels. ionic channels ca n be modified by the powerful techniques of of molecular biology. Charged g roups can be engineered (i.e. replaced one at a time) and the location of e very atom can be determined. channels can be studied in the tradition of el ectrochemistry. If coupled to the Poisson equation, the drift diffusion equ ations (i.e. Nernst-Planck equations) form an adequate model of the current through 6 different channel proteins with quite different characteristics in 10 solutions over +/-150 mV. In this theory the channel is represented a s a distribution of fixed charge, and the ion as a mobile charge with a dif fusion coefficient. The theory predicts the electric field (i.e. potential profile) and resulting current produced by the fixed charge and other charg es in the system. In this theory, the shape of the electric field is found to be a sensitive function of ionic conditions and the potential difference across the channel, in contrast to traditional theories that assume potent ial profiles (or rate constants) independent of experimental conditions. Tr aditional theories fail to fit data, probably for because they assume the s hape of the electric field. The Poisson-Nernst-Planck (PNP) theory is nearl y idemntical to the drift diffusion equations used to analyze the flow of q uasi-particles in semiconductors, implying that - given appropriate geometr y and profiles of fixed charge - ionic channels can perform many of the use ful functions of transistors, acting as resistors, voltage amplifiers, curr ent amplifiers, or logic elements. Channels form a useful system for electr ochemistry since they are biologically and clinically important, they follo w the simple rules of electrodiffusion, and they promise to be of considera ble use in technology (C) 2000 Elsevier Science B.V. All rights reserved.